A PON (Passive Optical Network) system is a conventional communication system formed from a parent station and child stations. The PON system performs communication by point-to-multipoint using an OLT (Optical Line Terminal) installed in a station building as a parent station and an ONU (Optical Network Unit) installed in each user home as a child station.
FIG. 1 shows the arrangement of a PON system 1. Referring to FIG. 1, the PON system 1 includes an OLT 2 installed in a station building, m ONUs 3 (3−1, 3−2, . . . , 3−m each installed in a user home, optical fibers 4 that connect the m ONUs 3 (3−1, 3−2, . . . , 3−m) to 1:m, and an optical splitter 5. Note that an external network 6 is connected to the OLT 2. The section where the ONUs 3 and the OLT 2 are connected by the optical fibers 4 and the optical splitter 5 will be referred to as a PON section 7.
In the PON system 1, signals transmitted from the plurality of ONUs 3 are bundled by the optical splitter 5 and reach the OLT 2 in this state. Hence, the PON system 1 defines the signal transmission timing of each ONU 3 not to cause confliction between the signals from the ONUs 3 (for example, see non-patent literatures 1 and 2).
The OLT 2 periodically transmits a reference time to each ONU 3 in accordance with the definition and notifies each ONU 3 of time at which it is permitted to transmit a signal. Each ONU 3 sets the local time of its own to the reference time received from the OLT 2, and when the local time has reached the designated time, transmits the signal and communicates with the OLT 2.
In the PON system 1, the ONU 3 is installed in each user home. For this reason, the power consumption of all ONUs 3 makes up a large proportion of the power consumption of the entire network, and the ONUs 3 are required to save power.
The ONU 3 uses, for example, a Cyclic Sleep method as the power saving method (see, for example, non-patent literature 3). In the Cyclic Sleep method, the ONU 3 has two modes, that is, a power saving mode and a normal mode. In the power saving mode, the ONU 3 performs Cyclic Sleep. In the normal mode, Cyclic Sleep is not performed.
In the Cyclic Sleep method, the ONU 3 periodically repeats a Sleep state and an Aware state in the power saving mode. The Sleep state indicates a state in which the apparatus is partially or wholly stopped to suppress power use. The Aware state indicates a state in which the apparatus is activated not to suppress power use. A period during which the ONU 3 is in the Sleep state will be referred to as a Sleep period, and a period during which the ONU 3 is in the Aware state will be referred to as an Aware period hereinafter. The time from the start of the Sleep period to the end of the Aware period will be referred to as one cycle. Signals transmitted/received between the OLT 2 and the ONUs 3 will be referred to as frames for distinction from internal signals of the apparatus. The frames include user frames and control frames. “User frame” is a general term for frames exchanged between the external network 6 and a home network, and “control frame” indicates frames (including a Sleep frame, Sleep_Ack frame, Aware frame, and Aware_Ack frame to be described later) other than the user frames.
FIG. 2 shows the communication process of the Cyclic Sleep method. In the Cyclic Sleep method, the OLT 2 instructs the ONU 3 to shift from the normal mode to the power saving mode or return from the power saving mode to the normal mode.
In actuality, referring to FIG. 2, the OLT 2 first decides, based on traffic and the like, to set a specific ONU 3 in the power saving mode, and transmits a control frame (in this case, Sleep frame) for instructing it to the ONU 3 at time of step ST1. Upon receiving the Sleep frame, the ONU 3 returns a control frame (in this case, Sleep_Ack frame) to the OLT 2 at time of step ST2 to notify it that the ONU 3 has acknowledged the shift to the power saving mode.
After transmitting the Sleep_Ack frame to the OLT 2, the ONU 3 is set in the Sleep state during a preset Sleep period (to be referred to as “T_sleep” hereinafter). When the Sleep period has ended, the ONU 3 is set in the Aware state during a preset Aware period (to be referred to as “T_aware” hereinafter). If no instruction is received from the OLT 2 until the end of the Aware period, the ONU 3 is set in the Sleep state during T_sleep again, and periodically repeats this operation from then on.
In the Cyclic Sleep method, to return the ONU 3 from the power saving mode to the normal mode, the OLT 2 sends a control frame (in this case, Aware frame) for instructing return from the power saving mode to the ONU 3 at time of step ST3. Upon receiving the Aware frame, the ONU 3 returns from the power saving mode to the normal mode, and transmits a control frame (in this case, Aware_Ack frame) to the OLT 2 at time of step ST4 to notify it of the return. From this point, the ONU 3 remains in the normal mode until a control frame (Sleep frame) is received from the OLT 2 again.
Note that in the Cyclic Sleep method, the time at which the Sleep period starts in the first cycle will be referred to as a power saving mode start time, and the OLT 2 and the ONU 3 are synchronized for the power saving mode start time.
FIG. 3 shows detailed examples of the arrangements of the OLT 2 and the ONU 3 that implement the Cyclic Sleep method. The OLT 2 includes a parent station communication unit 21, and (m) parent station power control units 22−1 to 22−m as many as the ONUs connected to the OLT 2.
The OLT 2 implements a protocol defined by non-patent literature 1 or 2 in the parent station communication unit 21. The OLT 2 maintains connection to each ONU 3 while periodically transmitting a control frame from the parent station communication unit 21 to the ONU 3 via the PON section 7 to notify them of the reference time, and transmits a user frame input from the external network 6 to the ONU 3 via the parent station communication unit 21 and the PON section 7. The OLT 2 also transmits a user frame, which is input from a home network 8 to the ONU 3 and transmitted from the ONU 3 via the PON section 7, to the external network 6 via the parent station communication unit 21.
The m parent station power control units 22 (22−1 to 22−m) of the OLT 2 correspond to the ONUs 3 connected to the OLT 2, and control whether the ONUs 3 should be in the power saving mode or the normal mode. In the OLT 2, each parent station power control unit 22 receives traffic Q1 of the corresponding ONU 3 from the parent station communication unit 21. The parent station power control unit 22 decides, based on the traffic Q1, whether to set the ONU 3 in the power saving mode. Each parent station power control unit 22 of the OLT 2 instructs the parent station communication unit 21 to transmit a control frame CF by a control signal C1. The parent station communication unit 21 generates the control frame CF such as the above-described Sleep frame or Aware frame in accordance with the transmission instruction and transmits the control frame CF to the ONU 3.
On the other hand, the ONU 3 includes a child station communication unit 31, a child station power control unit 32, and a child station period measurement unit 33. The ONU 3 implements the same communication protocol as that in the OLT 2 in the child station communication unit 31. The ONU 3 maintains connection to the OLT 2 while establishing time synchronization between the local time of the ONU 3 and the reference time of the OLT 2 based on the reference time transmitted from the OLT 2.
The ONU 3 transmits a user frame UF, which is input from the home network 8, from the child station communication unit 31 to the OLT 2. In addition, the ONU 3 transmits the user frame UF, which is input from the OLT 2 via the PON section 7, from the child station communication unit 31 to the home network 8. When the ONU 3 shifts to the Sleep state or the Aware state, the child station communication unit 31 of the ONU 3 controls stop or activation of the communication function by a stop/activation signal SPST input from the child station power control unit 32.
The child station power control unit 32 of the ONU 3 receives the control frame CF from the OLT 2 via the child station communication unit 31, and manages whether the ONU 3 should be in the power saving mode or the normal mode. More specifically, upon receiving the control frame (Sleep frame) CF from the OLT 2, the child station communication unit 31 of the ONU 3 notifies the child station power control unit 32 of the contents of the control frame (Sleep frame) CF by a control signal C2. As a result, the child station power control unit 32 of the ONU 3 shifts from the normal mode to the power saving mode based on the control signal C2 and repeats the Sleep state and the Aware state in a predetermined period. The child station period measurement unit 33 measures the Sleep period and the Aware period.
In the ONU 3, the child station power control unit 32 outputs a set signal SET and a reset signal RSET to the child station period measurement unit 33, and the child station period measurement unit 33 outputs a Sleep state signal SLM and an Aware state signal AWM to the child station power control unit 32. The set signal SET causes the child station period measurement unit 33 to start measuring the Sleep period and the Aware period. The reset signal RSET causes the child station period measurement unit 33 to stop the measurement. The Sleep state signal SLM is output in the Sleep period. The Aware state signal AWM is output in the Aware period.
In actuality, when the ONU 3 shifts from the normal mode to the power saving mode, the child station power control unit 32 outputs the set signal SET to the child station period measurement unit 33, and the child station period measurement unit 33 starts measuring the Sleep period and the Aware period. The ONU 3 causes the child station power control unit 32 to determine, based on the Sleep state signal SLM and the Aware state signal AWM from the child station period measurement unit 33, whether the ONU 3 is in the Sleep state or the Aware state and output the stop/activation signal SPST to the child station communication unit 31 to instruct stop/activation of the communication function.
In the above-described Cyclic Sleep method, however, the ONU 3 in the Sleep state stops the receiving function of the child station communication unit 31 and cannot receive the control frame (Sleep frame or Aware frame) CF from the OLT 2. For this reason, to return the ONU 3 from the power saving mode to the normal mode, the OLT 2 needs to transmit the control frame (Aware frame) CF of return instruction when the ONU 3 is in the Aware state.
To implement this, the OLT 2 needs to measure the Sleep period and the Aware period and grasp whether the ONU 3 is in the Sleep state or the Aware state. However, because of the clock deviation between the OLT 2 and the ONU 3, an error (Δt) occurs in the measurement of the Sleep period and the Aware period in the Sleep state in which clock synchronization cannot be established. Note that when the clock deviation of the apparatus is 100 ppm, the error (Δt) is about 1 μsec in a 10-msec Sleep period.
Even if small relative to the Sleep period or the Aware period, the error (Δt) accumulates in every cycle and increases to x×Δt in x cycles. For this reason, as shown in FIG. 4, after the cycle has repeated, a case may occur in which the OLT 2 is in the Aware period, whereas the ONU 3 is in the Sleep period. If the OLT 2 transmits the control frame (Aware frame) CF at this timing, the ONU 3 cannot receive it. Hence, the return from the power saving mode to the normal mode is impossible.
To avoid such a situation, the OLT 2 may continuously transmit the control frame (Aware frame) CF of return instruction at an interval much shorter than the Aware period. However, this method not only lowers the band utilization efficiency but also increases the load on the OLT 2 and thus increases the power consumption. Such a measure is necessary because the error (Δt) between the OLT 2 and the ONU 3 is not taken into consideration when measuring the Sleep period and the Aware period.
If the error (Δt) is too large to neglect relative to the Sleep period or the Aware period, the error (Δt) occurs between the OLT 2 and the ONU 3 during the Aware period and the Sleep period in the first cycle, and the same problem as described above arises. This is also because the error (Δt) is not taken into consideration when measuring the Sleep period and the Aware period.